The drug, sold under the name Sprycel, is already approved in the United States for chronic myelogenous leukemia. Experts said it's too early to say whether it should be added to the treatment arsenal for breast cancer, but they called the findings from this early study promising.

The study included 120 postmenopausal women with either a breast cancer recurrence or metastatic cancer -- meaning it had spread from the breast to the bones or other sites in the body.

Overall, women who received Sprycel along with hormonal therapy remained progression-free for twice as long as those given hormonal therapy alone: typically 20 months versus 10 months.

Experts said the findings, presented this week at the San Antonio Breast Cancer Symposium in Texas, are "encouraging." But more research needs to be done.

"It's a small study, and this is not ready for prime time yet," said lead researcher Dr. Dev Paul, a breast cancer specialist at U.S. Oncology and Rocky Mountain Cancer Centers in Denver.

He said a big question is, can doctors find a way to identify the particular women who stand to benefit from Sprycel? The drug targets a protein called Src (pronounced "serk"). And it's thought that in some women activity in that protein contributes to the spread of breast cancer to the bones.

But right now, there's no known indicator, or "biomarker," in the blood that can be measured to gauge a patient's Src activity, Paul said.

Dr. Elias Obeid, a breast cancer specialist at Fox Chase Cancer Center in Philadelphia, agreed on the need for a Src indicator.

Women with advanced breast cancer can live for several years with current treatments. And for any one woman, Obeid explained, you have to weigh the potential benefits of a new treatment against the risks.

"That balance is very important," said Obeid, who was not involved in the study. "We have to be looking at quality of life."

In this study, close to 40 percent of women on Sprycel had fatigue or nausea. About one-quarter had anemia or rash, and 16 percent had fluid buildup around the lungs, which can cause chest pain and breathlessness, and require treatment to drain the fluid.

"Those side effects can be significant," Obeid noted.

Sprycel is among the wave of new cancer drugs designed to be "targeted" therapies. They zero in on particular proteins in cancer cells, which, it's hoped, will allow them to be more effective and less toxic than conventional chemotherapy.

The women in the current study all had hormone receptor-positive breast cancer, which means estrogen and progesterone help fuel the cancer's growth. Most cases of breast cancer (about 75 percent) are hormone-sensitive.

Hormone-blocking drugs, such as tamoxifen, are the standard treatment for recurrent or metastatic breast cancer. "Most women initially respond to the hormone therapy," Paul said. "But eventually they become resistant."

It's thought that activity in the Src protein is one way the cancer develops resistance and continues to progress. So Paul's team reasoned that adding Sprycel might boost the effectiveness of hormone therapy.

Of the women in the study, 63 were randomly assigned to standard hormone therapy with the drug letrozole (Femara), and 57 to Femara plus Sprycel.

Sprycel actually brought no extra benefit when it came to the number of women who saw their cancer shrink, or at least had no progression for six months or more. More than 60 percent of women in each group had those responses.

The difference was seen in "progression-free survival" -- the typical amount of time the women showed no cancer progression -- which was 10 months longer for women on Sprycel.

While that's an encouraging finding, "it's too early to jump on this," said Dr. Subhakar Mutyala, associate director of the Scott & White Cancer Institute in Temple, Texas.

Mutyala, who was not involved in the study, agreed that because women with metastatic breast cancer can live for a number of years, the side effects of any new treatment are "really important" to consider.

"Right now, we can say dasatinib [Sprycel] shows activity in these patients," Mutyala said. But the drug's ultimate benefit for women's lives is not clear, he added.

And like other targeted cancer drugs, such as Gleevec and Tasigna, Sprycel rings up at around $100,000 or more a year in the United States.

Bristol-Myers Squibb, which markets Sprycel, funded the study. Paul said he has no financial conflicts of interest.